1. Field of the Invention
The present invention relates to a method of forming a superconductive thin film by using a physical vapor deposition method, and more particularly, to a method of forming a superconductive thin film having a high critical temperature of 87K or higher and having a thickness of 500 nm or larger.
2. Description of the Related Art
There has so far been provided with a high temperature superconductor which is called as an yttrium, barium, and copper oxide system (hereinlater referred to simply as a "YBCO system"). The YBCO system superconductor has a composition represented by "Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y ". The subscript character "y" attached to the chemical symbol "O" represents the oxygen content of the YBCO system superconductor. The oxygen content of the YBCO system superconductor is varied between 6 and 7. In the YBCO system superconductor thus composed, there is caused, at a critical temperature, superconductive phenomena in which the superconductor has an extremely low electric resistance. The critical temperature of the YBCO system superconductor is much higher than the boiling point of liquid nitrogen.
As is well known, the YBCO system superconductor forms crystals in either tetragonal system or orthorhombic system. There is caused a phase transition between the tetragonal system and the orthorhombic system in the crystals in accordance with the oxygen content of the composition. When the superconductor contains insufficient oxygen in the composition, i.e. when the oxygen content y.ltoreq.6.5, the superconductor forms the tetragonal system crystal and has a critical temperature of lower than or equal to 50K. When the superconductor contains sufficient oxygen in the composition, i.e. when the oxygen content y.ltoreq.6.5, the superconductor forms the orthorhombic system crystal and has a critical temperature between 80K and 87K. That is to say that the superconductor in the orthorhombic system crystal has an extremely high critical temperature in comparison with that of the superconductor in the tetragonal system crystal.
The above YBCO system superconductor is preferably utilized for an integrated circuit, because of the fact that the superconductor has an advantage over a normal metal conductor in reducing transmission loss of signals and in enhancing the frequency response characteristics of the integrated circuit. In this instance, the YBCO system superconductor is generally formed into a thin film having a thickness of several hundred nanometers.
In general, the superconductive thin film which is utilized for fabricating the integrated circuit is deposited on the substrate through a physical vapor deposition method (hereinlater referred to as "PVD method" for brevity), e.g., a sputtering method or a reactive vapor deposition method.
The conventional superconductive thin film formation method will be explained hereinlater with referring to FIG. 8 of the drawings. The superconductive thin film formation method comprises processes of depositing the superconductive thin film and of introducing oxygen. As shown in FIG. 8, the temperature of the substrate is varied as time lapses in the formation of the superconductive thin film. The above processes are denoted by the symbols "Pa" and "Pb", respectively, in FIG. 8.
The deposition process Pa comprises the steps of: heating the substrate at a first substrate temperature Ta of 650.degree. C. or higher; and depositing the superconductor on the substrate under a first oxygen partial pressure of 1 Torr or lower. As shown in FIG. 8, the deposition process Pa is carried out and continues for a deposition process time ta. Thus formed superconductive thin film has a thickness of 500 nm or smaller and a critical temperature of 50K or lower. The reason why the superconductive thin film has the low critical temperature is considered that the superconductive thin film dose not contain sufficient oxygen in the composition and forms tetragonal system crystal.
In order to change the superconductive thin film in phase from the tetragonal system crystal to the orthorhombic system crystal, the oxygen introduction process Pb is carried out. The oxygen introduction process Pb is carried out through an annealing treatment under the condition of a second oxygen partial pressure of 10 Torr or higher and a second substrate temperature Tb of 500.degree. C. or lower. The second oxygen partial pressure is higher than the first oxygen partial pressure. The second substrate temperature Tb is lower than the first substrate temperature Ta.
The oxygen introduction process Pb follows the deposition process Pa and is carried out and continues for an oxygen introduction process time tb. After the oxygen introduction process Pb, the superconductive thin film has a critical temperature of 80-87K. As described above, before the oxygen introduction process Pb, the superconducting thin film has the critical temperature of 50K or lower. It is understood from this result that the critical temperature after the process Pb is higher than the critical temperature before the process Pb.
In the above conventional method, the superconductive thin film attains the various thicknesses in accordance with the process times. For instance, each of the processes Pa and Pb is carried out and continues for about 6 hours to attain the thickness of 450 nm. Furthermore, each of the processes Pa and Pb is carried out and continues for about 12 hours to attain the thickness of 900 nm.
The superconductive thin films having the thicknesses of 450, 600 and 900 nm formed through the conventional method have the critical temperatures of 87, 84 and 80K, respectively. This means that as the thickness of the superconductive thin film becomes larger, the critical temperature of the superconductive thin film becomes lower.
A drawback is, however, encountered in a prior-art superconductive thin film formation method of the above described nature in that it is impossible to form the superconductive thin film having a thickness of 500 nm or larger and a critical temperature of 87K or higher.